The present invention relates in general to processors for the periphonic reproduction of Sound. More specifically, the invention relates to improvements in the detector filter, separation matrix, low frequency center front cancellation circuit, servologic control voltage generator, voltage controlled amplifiers, and output shelf filter circuits of a surround sound processor for multichannel redistribution of audio signals.
A surround sound processor operates to enhance a two-channel stereophonic source signal so as to drive a multiplicity of loudspeakers arranged to surround the listener, in a manner to provide a high-definition soundfield directly comparable to discrete multitrack sources in perceived performance. An illusion of space may thus be created enabling the listener to experience the fullness, directional quality and aural dimension or "spaciousness" of the original sound environment. The foregoing so-called periphonic reproduction of sound can distinguished from the operation of conventional soundfield processors which rely on digitally generated time delay of audio signals to simulate reverberation or "ambience" associated with live sound events. These conventional systems do not directionally localize sounds based on information from the original performance space and the resulting reverberation characteristics are noticeably artificial.
To accomplish this end, a surround sound processor typically comprises an input matrix, a control voltage generator and a variable matrix circuit.
The input matrix usually provides for balance and level control of the input signals, generates normal and inverted polarity versions of the input signals, plus sum and difference signals, and in some cases generates phase-shifted versions, and/or filters the signals into multiple frequency ranges as needed by the remainder of the processing requirements.
The control voltage generator includes a directional detector and a servologic circuit. The directional detector measures the correlations between the signals which represent sounds encoded at different directions in the stereophonic sound stage, generating voltages corresponding to the predominant sound location. The servologic circuit uses these signals to develop control voltages for varying the gain of voltage-controlled amplifiers in the variable matrix circuit in accordance with the sound direction and the direction in which it is intended to reproduce the sound in the surrounding loudspeakers.
The variable matrix circuit includes voltage-controlled amplifiers and a separation matrix. The voltage-controlled amplifiers amplify the input matrix audio signals with variable gain, for application to the separation matrix, where they are used to selectively cancel crosstalk into different loudspeaker feed signals. The separation matrix combines the outputs of the input matrix and of the voltage-controlled amplifiers in several different ways, each resulting in a loudspeaker feed signal, for a loudspeaker to be positioned in one of several different locations surrounding the listener. In each of these signals, certain signal components may dynamically eliminated by the action of the detector, control voltage generator, voltage-controlled amplifiers (VCA's) and separation matrix.
In surround sound processors, much of the subtleties of the presentation are due to the characteristics of direction detector and servologic circuit of the control voltage generator and of the VCA's. As these are further refined, the apparent performance becomes more transparent and effortless-sounding to the listener.